Quantitative description of carrier dynamics in GaSb/GaAs quantum-ring-with-dot structures

Abstract

Self-assembled GaSb/GaAs quantum-ring-with-dot structures (QRDSs) are the nanostructures exhibiting type-II band alignment. Each QRDS consists of both quantum ring (QR) and quantum dot (QD) parts which have their own energy levels. In this work, the carrier dynamics in the GaSb/GaAs QRDSs are explored and quantitatively described by a rate equation model which is developed from experimental photoluminescence (PL) spectra in literature. The model is comprised of the carrier transition rates and activation energies involved the transfer processes of thermal-excited carriers. The difference between the QR and QD PL intensities is also taken into account in the model. Electronic structures of a single QRDS are calculated in order to determine the overlap between electron and hole wave functions that can be used for estimating the radiative transition rates. Numerical values of the radiative and non-radiative transition rates, carrier capture and escape rates, and activation energies are presented. The PL spectra obtained from the model are consistent with the reported experimental data.

Optical absorption spectra of GaSb/GaAs quantum-ring-with-dot structures and their potential for intermediate band solar cells

“…According to the atomic force microscopy images of real QRDSs [25], a model QRDS is considered as a combination of an upper-half flat torus and an upper-half oblate spheroid, where the QR and QD bases are aligned [26], as shown in figure 1(a). In this study, the QR part before combining with the QD one is described by the equations x(θ, φ) = (R + r cos θ) cos φ, y(θ, φ) = (R + r cos θ) sin φ and z(θ, φ) = h R |sin θ| for 0 ⩽ θ, φ < 2π where the major radius (R), minor radius (r) and height (h R ) are 30, 15 and 1 nm, respectively [26]. The QD part is defined by the equations x(θ, φ) = (D/2) sin θ cos φ, y(θ, φ) = −R + (D/2) sin θ sin φ and z(θ, φ) = h D |cos θ| for 0 ⩽ θ, φ < 2π where the base diameter (D) and height (h D ) are 40 and 1.8 nm, respectively [26].…”

“…In this study, the QR part before combining with the QD one is described by the equations x(θ, φ) = (R + r cos θ) cos φ, y(θ, φ) = (R + r cos θ) sin φ and z(θ, φ) = h R |sin θ| for 0 ⩽ θ, φ < 2π where the major radius (R), minor radius (r) and height (h R ) are 30, 15 and 1 nm, respectively [26]. The QD part is defined by the equations x(θ, φ) = (D/2) sin θ cos φ, y(θ, φ) = −R + (D/2) sin θ sin φ and z(θ, φ) = h D |cos θ| for 0 ⩽ θ, φ < 2π where the base diameter (D) and height (h D ) are 40 and 1.8 nm, respectively [26]. The GaSb layer is formed by the Sb-for-As exchange reaction during the crystallization process of droplet epitaxy [23] and has a thickness of ∼1 nm [8,26].…”

“…For the GaSb/GaAs material system, common NSs such as QDs and QRs as well as unusual NSs such as quantum-ring-with-dot structures (QRDSs) can be grown by droplet epitaxy [22][23][24][25]. Although the morphology, photoluminescence (PL) and carrier dynamics of GaSb/GaAs QRDSs have already been explored [25,26], their promising applications have never been mentioned. Each QRDS consists of QR and QD parts (see figure 1(a)).…”

“…Each QRDS consists of QR and QD parts (see figure 1(a)). Since the QRDS ensemble exhibits a bimodal height distribution [25] corresponding to the two PL emission bands derived from the confined states in the QR and QD parts [25,26], the QRDSs are expected to provide two IBs in the bandgap of GaAs host material. In addition to the carrier dynamics, the optical absorption of the QRDSs must be known in order to determine whether the QRDSs have the potential for the IBSC applications.…”

“…The absorption coefficients in the cases of the single QRDS layer and multi-stacked QRDS layers are presented and discussed. Based on the calculated absorption spectra and the carrier dynamics in the literature [26], a drift-diffusion model coupled with rate equations for a solar cell containing multistacked QRDS layers is formulated. Interestingly, the results from the model show that the confined hole states in the QRs effectively function as the IB in the solar cell.…”